1. TECHNICAL FIELD
The present invention relates to an organic halogen compound decomposition device which utilizes a plasma, a decomposition method for organic halogen compounds using the device, and an operation control method for the above decomposition device. In particular the invention relates to an organic halogen compound decomposition device which uses microwaves to generate a plasma, a microwave plasma torch and a plasma ignition method suitable for use in the decomposition device, and an operation control method for the above decomposition device.
2. BACKGROUND ART
Organic halogen compounds such as freon, trichloromethane and halon, which incorporate elements such as fluorine, chlorine and bromine within the molecule, are used in large quantities for a variety of uses such as refrigerants, solvents and fire extinguishing agents, and are extremely important in industry.
However, these compounds display high volatility, and if discharged untreated into the environment, such as into the atmosphere, soil, or water, can have undesirable effects on the environment such as the generation of carcinogenic substances or the destruction of the ozone layer. Consequently from an environmental conservation viewpoint, it is necessary to treat the compounds to render them harmless.
Conventional treatment methods for organic halogen compounds typically utilize thermal decomposition reactions at high temperatures. These treatment methods can be further roughly classified into incineration methods and plasma methods.
In incineration methods, organic halogen compounds are incinerated together with typical waste products such as plastics. In contrast, in plasma methods, the organic halogen compound is reacted with steam within a plasma, and decomposed to carbon dioxide, hydrogen chloride and hydrogen fluoride. In recent years, operation control methods have been developed for organic halogen compound decomposition devices which utilize the above plasma methods and which utilize microwaves to generate the plasma.
Decomposition devices used in such decomposition methods comprise an exhaust gas treatment tank containing an alkali solution, a reaction tube in which the open lower end section thereof is immersed in the alkali solution, a cylindrical waveguide which extends in a vertical direction above the reaction tube, a discharge tube disposed inside the cylindrical waveguide in which the bottom end thereof interconnects with the reaction tube, a square waveguide in which one end thereof is connected to the cylindrical waveguide, and a microwave transmitter which is installed at the other end of the square waveguide.
In this type of decomposition device, freon gas and steam are supplied to the discharge tube, while microwaves generated by the microwave transmitter are transmitted to the cylindrical waveguide via the square waveguide. Then, a discharge is produced within the microwave electric field formed inside the cylindrical waveguide, and the freon gas is decomposed by a thermal plasma within the reaction tube. The product gases produced by this decomposition reaction are then neutralized by passage through the alkali solution, and any remaining gases such as carbon dioxide are expelled through an exhaust duct.
However, in the above plasma type organic halogen compound decomposition device, prior to the formation of the plasma, it is first necessary to ignite the gas in a stable and reliable manner enabling a simple formation of the plasma. Furthermore, in terms of the operation of the device following ignition, maintenance of a good plasma form and maintenance of a stable decomposition reaction are important factors.
Moreover, in the above plasma type organic halogen compound decomposition device, because the freon gas or the like needs to be in a plasma state, the device needs to be controlled so that the freon gas is converted to a plasma in a stable and reliable manner. Furthermore, because the substances generated on decomposition of the freon gas are toxic, safety must also be ensured in the control of the device.
The present invention takes the above factors into consideration, with an object of providing a microwave plasma torch and an organic halogen compound decomposition device for which a stable and reliable ignition is possible prior to plasma formation, and moreover for which good plasma form, and stable operation, can be maintained following ignition.
Furthermore, another object of the present invention is to provide a plasma ignition method which enables a stable and reliable ignition prior to the plasma formation, and an organic halogen compound decomposition method which utilizes this ignition method.
Moreover, another object of the present invention is to provide an operation control method for an organic halogen compound decomposition device in which an organic halogen compound such as a freon gas can be converted to a plasma in a stable and reliable manner. In addition, another object of the present invention is to provide an operation control method for an organic halogen compound decomposition device which enables an improvement in safety of the organic halogen compound decomposition devices.
In order to resolve the above problems, the following construction was employed in the present invention.
A microwave plasma torch of the present invention comprises a cylindrical waveguide made up of an outer conductor and an inner conductor, and a dual construction discharge tube made up of an inner tube and an outer tube which is provided inside the cylindrical waveguide and on an identical axis, wherein a probe antenna formed from an extension of the inner conductor is positioned so as to encircle the discharge tube, and the tip of the inner tube is positioned further inwards than the tip of the probe antenna.
According to this microwave plasma torch, the helical gas flow flowing out from between the inner tube and outer tube forms a stagnation point directly below the inner tube. Consequently, the gas flow stagnation point can be easily ignited, making a stable and reliable ignition possible. Furthermore, because the tip of the inner tube is positioned further inwards than the tip of the probe antenna, the inner tube tip section is separated from the energy intensive section of the plasma, and is less likely to be subjected to heating effects.
Moreover, because a distance L is provided between the inner tube tip and the probe antenna tip, with the inner tube being positioned further inwards, the inner tube tip is separated from the energy intensive section of the plasma, and is less likely to be subjected to heating effects. Consequently, thermal deformation of the inner tube can be prevented, and by providing a stable gas flow, a stable decomposition reaction can be maintained.
Furthermore, an organic halogen compound decomposition device of the present invention comprises an exhaust gas treatment tank containing a treatment solution for neutralization treatment of the decomposition products from the organic halogen compound, a reaction tube in which the open lower end section thereof is immersed in the treatment solution of the exhaust gas treatment tank, and a microwave plasma torch, and the microwave plasma torch further comprises a cylindrical waveguide made up of an outer conductor and an inner conductor, which extends in a vertical direction above the reaction tube, and a dual construction discharge tube made up of an inner tube and an outer tube which is provided inside the cylindrical waveguide and on an identical axis, wherein a probe antenna formed from an extension of the inner conductor is positioned so as to encircle the discharge tube, and the tip of the inner tube is positioned further inwards than the tip of the probe antenna.
This organic halogen compound decomposition device employs a plasma torch which has excellent ignition properties and which is able to maintain a stable decomposition reaction, and so at the beginning of operation, a stable and reliable ignition is possible. Moreover, a good plasma form is maintained during operation, thereby enabling the maintenance of a stable decomposition reaction.
Furthermore, a plasma ignition method of the present invention is a plasma ignition method in which argon gas, which is introduced into a dual structure discharge tube comprising an inner tube and an outer tube and which flows in a swirling motion between the inner tube and the outer tube prior to plasma generation, is ignited to form a plasma, wherein the axial flow rate of the argon gas flowing between the inner tube and the outer tube is set to a value of at least 80 cm/second.
This plasma ignition method, by employing a dual structure discharge tube and using a high velocity argon gas axial flow rate of at least 80 cm/second, generates a suitable stagnation point in the gas flow in the lower section of the inner tube, and by conducting ignition at this stagnation point, a very high ignition ratio of greater than 90% is achieved at the time of plasma formation, making a stable and reliable ignition possible. Moreover, the aforementioned stagnation point maintains the plasma in a stable state even following ignition, and so also contributes to the maintenance of a good plasma.
Furthermore, an organic halogen compound decomposition method of the present invention is an organic halogen compound decomposition method which uses an organic halogen compound decomposition device comprising an exhaust gas treatment tank containing an alkali solution, a reaction tube in which the open lower end section thereof is immersed in the alkali solution, a cylindrical waveguide which extends in a vertical direction above the reaction tube, a dual structure discharge tube comprising an inner tube and an outer tube which is disposed inside the cylindrical waveguide and in which the bottom end thereof interconnects with the reaction tube, a square waveguide which extends horizontally and in which one end thereof is connected to the cylindrical waveguide, and a microwave transmitter which is installed at the other end of the square waveguide, and in which an organic halogen compound is reacted with steam inside the reaction tube and is decomposed by a plasma, wherein prior to formation of the plasma, the axial flow rate of the argon gas, which is introduced into the discharge tube and flows in a swirling motion between the inner tube and the outer tube, is set to a value of at least 80 cm/second, and the argon gas is then ignited to form the plasma.
This organic halogen compound decomposition method employs a stable and reliable ignition method, and so a very high ignition ratio of greater than 90% is achieved at the time of plasma formation, making a stable and reliable ignition possible. Furthermore, maintenance of the plasma following ignition is also easier, and so a stable decomposition reaction can be easily maintained.
Moreover, according to this organic halogen compound decomposition method, it is preferable that the aforementioned axial flow rate is set to a value of at least 230 cm/second.
According to this organic halogen compound decomposition method, by setting the axial flow rate at this higher velocity, the ignition ratio reaches 100%, making an even more stable and reliable ignition possible.
Furthermore, an operation control method for an organic halogen compound decomposition device according to the present invention is an operation control method for an organic halogen compound decomposition device in which an organic halogen compound is converted to a plasma and reacted with water to decompose the organic halogen compound, wherein a rare gas and then the organic halogen compound are supplied in sequence to the device system, and the rare gas is converted to a plasma before supply of the organic halogen compound is commenced, and following commencement of the supply of the organic halogen compound, supply of the rare gas is stopped.
According to this operation control method for an organic halogen compound decomposition device, first a rare gas which is easily converted to a plasma state, such as AR, NE or He, is converted to a plasma, and an organic halogen compound is subsequently supplied and the organic halogen compound also converted to a plasma. Then, once the plasma state of the organic halogen compound has stabilized, the supply of the rare gas is stopped. That is, according to this operation control method for an organic halogen compound decomposition device, a time overlap in the supply of the rare gas and the organic halogen compound can be provided, and as a result, the plasma of the organic halogen compound can be generated in a stable and reliable manner, and moreover consumption of the rare gas can be limited. The time overlap in the supply of the rare gas and the organic halogen compound may be extremely short. Consequently, during the long period of the organic halogen compound decomposition, the supply of the rare gas is unnecessary.
Moreover, according to this operation control method for an organic halogen compound decomposition device, it is preferable that the conversion to plasma is performed by irradiation with microwaves.
In comparison with high frequency induction plasmas, plasmas generated by microwave irradiation have more stable plasma flames. Consequently, even if supply of the rare gas, which is readily converted to a plasma, is stopped, there is almost no effect on the conversion of the organic halogen compound to plasma form. As a result, once the conversion to plasma of the organic halogen compound has stabilized, even if supply of the rare gas is stopped the organic halogen compound can still be converted to a plasma state in a stable and reliable manner.
In addition, according to this operation control method for an organic halogen compound decomposition device, supply of water should preferably be commenced after supply of the rare gas, and before supply of the organic halogen compound is commenced.
In this method, first supply of the rare gas which is readily converted to a plasma is commenced, and the rare gas converted to a plasma. Then, an organic halogen compound to undergo decomposition is supplied to the device system, and by reacting the organic halogen compound in a plasma state with steam, acidic gases are generated as decomposition products, although supply of the water must be commenced prior to supply of the organic halogen compound. The reason for this requirement is that if only the organic halogen compound is converted to a plasma state, then recombination of dissociated atoms generates unexpected toxic halogen compounds, making it impossible to conduct processing to render the products harmless. In other words, with this method, because water is present before the decomposition of the organic halogen compound commences, the organic halogen compound and the water react, enabling the organic halogen compound to be decomposed safely. Furthermore, because the water supply is started after the rare gas supply, the inside of the device system can be kept dry, thereby enabling the ignition to be stabilized.
In addition, according to this operation control method for an organic halogen compound decomposition device, it is preferable that prior to commencement of the supply of the rare gas, a moisture removal treatment is performed on the inside of the device system.
During operation of the organic halogen compound decomposition device, if residual moisture is present inside the device system, then ignition of the plasma will not be stable. Consequently, by removing any moisture from within the device system before plasma ignition, ignition of the plasma can be performed in a stable and reliable manner.
In such a case, the moisture removal process performed on the inside of the device system to ensure stabilization of the plasma ignition should preferably be conducted by supplying a gas to the system. In particular, by for example, heating the supplied gas to a temperature of 100xcx9c800xc2x0 C., or drying the gas, any residual moisture inside the system can be effectively removed.
Moreover, according to this operation control method for an organic halogen compound decomposition device, it is preferable that following the stopping of the supply of the organic halogen compound, a scavenging gas is supplied into the device system.
By passing a scavenging gas through the device system following the stopping of the supply of the organic halogen compound, that is, following completion of the decomposition of the organic halogen compound, any residual acidic gases produced as decomposition products inside the system are purged, thereby improving the safety of the system.
Furthermore, an operation control method for an organic halogen compound decomposition device according to the present invention is an operation control method for an organic halogen compound decomposition device in which an organic halogen compound is decomposed by conversion to a plasma and subsequent reaction with water, wherein decomposition treatment of the organic halogen compound is performed following treatment to remove moisture from within the device system.
According to this operation control method for an organic halogen compound decomposition device, by removing any moisture from inside the device system, which is a major cause of instability in the plasma ignition, before decomposition treatment of the organic halogen compound is commenced, the plasma ignition can be conducted in a stable and reliable manner.
Furthermore, an operation control method for an organic halogen compound decomposition device according to the present invention is an operation control method for an organic halogen compound decomposition device in which an organic halogen compound is decomposed by conversion to a plasma and subsequent reaction with water, wherein following completion of the decomposition treatment, a scavenging gas is supplied into the device system.
According to this operation control method for an organic halogen compound decomposition device, by supplying a scavenging gas into the device system following completion of the decomposition of the organic halogen compound, any residual acidic gases inside the system are purged, thereby improving the safety of the system.
Moreover, in the aforementioned operation control method for an organic halogen compound decomposition device, it is preferable that during decomposition of the organic halogen compound, the treatment solution used for neutralizing the decomposition products from the organic halogen compound is stirred, and that following completion of the decomposition, the stirring is stopped.
Stirring the treatment solution during decomposition of the organic halogen compound, facilitates the treatment of decomposition products. Furthermore, stopping the stirring following completion of the decomposition, allows reaction products within the treatment solution to settle, and also contributes to power conservation. Moreover, commencement and stopping of the stirring process need not necessarily match the commencement and stopping of the decomposition process.